After the fall of Saigon on April 29, 1975, military and civilian strategists sought “lessons learned.” Many were tactical or technical, such as the operational effectiveness of precision-guided munitions and the continuing need for guns on jet fighters. At the strategic level, one pundit recommended that the United States never again fight in a former French colony located on the other side of the world with borders contiguous to enemy sources of supply governed by an ally of dubious political legitimacy. After the fall of Saigon 37 years ago, the United States embarked on another unsatisfying war, the result seeming eerily familiar. What was missed in post-Vietnam assessments that might have informed a strategically efficacious approach to the War on Terror?

First, understand the historical context. The Vietnam intervention resulted from a Cold War mindset that assumed the war in South Vietnam was part of a larger “communist plot for world domination.” That made Vietnam more important than it was. The resulting intervention into a local struggle tied U.S. prestige to a dubious cause. Lesson: Look closely at the local situation before commitments become irrevocable.

Second, there are dangers in incrementalism. It is a myth that the United States “blundered” into a Vietnam quagmire. American intervention resulted from a series of small, incremental steps, each seemingly low in risk. By the end of 1965, with over 100,000 American service personnel committed to Vietnam, the U.S. presence was hostage to a faulty policy. The political cost of getting out seemingly outweighed the military cost of staying in.

Third, there are limits to what military power can achieve. In 1961, when the Kennedy administration decided to “draw a line in the sand” in Vietnam, the general military assumption was that U.S. military power, sufficient to defeat Nazi Germany, fascist Italy, and imperial Japan in less than four years, could easily handle an insurgency in South Vietnam supported by an impoverished military power in North Vietnam. Surely a nation reaching toward outer space had little to fear from a country where few people knew how to drive a car.

History shows that small nations and dedicated movements can defeat major powers. England defeated the Spanish Empire in the 16th century. The American Revolution succeeded against the British Empire. Japan defeated Russia in 1905.

In March 2003, with Operation Iraqi Freedom, the assumption was U.S. forces would be in Baghdad within a month. It took three weeks. Then the real war started and U.S. forces languished there for the next eight years.

Alabama football coach Paul “Bear” Bryant understood, “It’s not the size of the dog in the fight, it’s the size of the fight in the dog that counts.”

Fourth, know your enemy. From the start of the Vietnam War, the fatal assumption was that Hanoi and the National Liberation Front—the Viet Cong—could be coerced with incrementally applied force. Their goals were not amenable to our logical frames of reference. The North Vietnamese and the Viet Cong were willing to pay an enormous price for victory.

The “War on Terror” suffered from the failure to identify the enemy as Islamist fundamentalist-Jihadists determined to defeat the United States and, ultimately, bring down Judeo-Christian civilization. Knowing yourself corresponds with knowing the enemy.

Fifth, Americans are not patient. In 1946, General of the Army George C. Marshall stated, “America cannot fight a Seven Years’ War.” In 1968, the Tet Offensive occurred almost precisely seven years after the Kennedy administration drew the line in Vietnam. Frustrations grew throughout the subsequent administrations of Lyndon Johnson and Richard Nixon, weakening public will.

Sixth, beware of open-ended commitments to regimes of dubious legitimacy. In Vietnam, first the United States committed its power and prestige to the support of Ngo Dinh Diem, a self-described “16th-century Spanish Catholic” who governed like a mandarin in an overwhelmingly Buddhist country struggling to throw off its colonial past. When in late 1963, Diem proved ineffective, the United States acquiesced in a coup resulting in a succession of military dictators.

History’s not so tidy that mistakes in the War on Terror are entirely analogous to those in Vietnam. The current war proceeded with an all-volunteer force, not a conscript-driven force. From October 2001 to the present, American military leadership, at every level, has been outstanding. The Bush administration’s big mistake was not clearly identifying the enemy. The Obama administration’s blunder was to set a deadline for withdrawal.

Wars are the most unpredictable of human endeavors, fraught with the unexpected and quite often, when strategically ill-conceived, much longer and bloodier than anticipated. That’s why over 2,000 years ago, Sun Tzu wrote, “War is a matter of vital importance; the province of life or death; the road to survival or ruin. It is mandatory that it be thoroughly studied.”